Detecting concealed weapons has been a major concern of law enforcement for decades, but today's most widely used technology, metal detectors, only provide a partial solution. Metal detectors can only detect weapons that produce magnetic signals above a certain threshold, so non-metallic weapons or small metal objects like box cutters and razor blades often go undetected. Even worse, as the distance between the sensor and the target increases, even large objects like guns only produce faint signals.
While there are a number of alternative concealed weapons detection (CWD) technologies in development that could overcome this problem, each has its own limitations. In some cases, such as ultrawideband millimeter wave or terahertz imaging, the cost of the system is in the $100,000 range and the resulting anatomically accurate images raise serious privacy concerns. Other systems, such as magnetic gradiometers, are more sensitive than current metal detectors and competitively priced. However, they still can only detect weapons made of ferromagnetic materials. [1]
Acoustic sensing has received comparatively little attention as an alternative CWD technology, even though it could be a cost-competitive alternative to metal detectors, and can detect weapons of any material composition. In addition, privacy and safety are not problems with acoustics because acoustic imaging does not provide precise anatomical detail of the person scanned, and because it is safe.
Felber et al. [2-4], developed an air-coupled acoustic imager that could detect characteristic “glints” of weapons concealed under heavy clothing at ranges up to 15 feet with a 40 kHz ultrasonic wave. This prototype employed a single, focused ultrasound transducer operating in a pulse-echo imaging mode. Unfortunately, the prototype also displayed a low signal-to-clutter ratio (caused by reflections from clothing) and was very sensitive to changes in the orientation of the weapon (normal incidence reflections produce the largest signals). As a result, the prototype had a high false alarm rate and a low probability of detection. A different device built by Jaycor operates at a frequency of about 70 to 100 kHz, and can resolve objects to about 4 to 6 inches at distances of 10 to 15 feet [5]. Such resolution can still allow smaller dangerous items to go undetected. Whether the issues associated with the probability of detection and false alarm rate have been resolved is unclear.
These shortcomings of traditional pulse-echo imaging are the result of some fundamental properties of ultrasonic waves. Air-coupled ultrasound detectors must operate at frequencies that can reach the subject, define an interrogation area, and penetrate clothing. These are inverse requirements: high frequency is needed to define a small spot size yet low frequency is needed to penetrate clothing. In fact, Nacci and Mockensturm [6] observe that although ultrasound systems are less expensive than radar, “ultrasound does not penetrate clothing as well as radar.”
Ultrasonic waves are often used in medical applications, both as an imaging technology and for therapeutic use. Fatemi and Greenleaf [7-8] developed a technique for medical applications called vibro-acoustography or ultrasound-stimulated acoustic emission. In this approach, acoustic energy was emitted from solids and tissues in response to an oscillatory radiation force produced by interfering focused beams of ultrasound. A series of patents and patent applications by these innovators [9-13] disclose various medical applications that exploit modulation of an ultrasonic wave and/or interfering ultrasonic waves to produce non-linear effects. Note that in typical medical applications, the ultrasonic transducer is either coupled directly to the body to be probed or the coupling is accomplished through the use of a liquid or gel intermediary. Air coupling would result in significant loss of ultrasonic energy at the air-body interface.
Additional medical applications are discussed by Averkiou [14-15] for performing non-linear echo signal imaging and to detect features excited by higher harmonics. Doukas [16] reports the therapeutic use of non-linear ultrasonic effects. In the examples given, the non-linearly produced waves are also in the ultrasonic frequency range.
In air, ultrasonic waves experience much stronger absorption than audible acoustic waves. However, given sufficiently strong ultrasonic waves, the non-linear effects can be used to advantage. Norris [17] developed a system for generating audible acoustic waves through the principle of acoustical heterodyning. His approach comprises two ultrasonic frequency transducers that are oriented so as to cause interference between emitted ultrasonic wave trains. When the difference in frequency between the two ultrasonic wave trains is in the audible frequency range, a new, audible wave train emanates outward from within the region of heterodyning interference. A different embodiment of the system employs parametric generation of the ultrasonic waves from a single ultrasonic direct radiating element. A later patent by Norris and Croft [18] further exploits the parametric version together with a reflective environment to form a surround-sound system.
Pompei [19] discusses the development of versatile and efficient transducers that are suitable for parametric as well as other ultrasonic applications. His transducers overcome previous transducer performance limitations that have inhibited the further development of parametric loud-speakers.
In summary, concealed weapons that contain metal can be detected easily with magnetometers from a close range such as an airport portal or a hand wand. Non-metallic weapons and concealed weapons at a distance are difficult to detect. While there are a number of alternative CWD technologies in development that could overcome this problem, each has its own limitations. In some cases, such as ultrawideband, millimeter wave or terahertz imaging, the cost of the system is in the $100,000 range and the resulting anatomically accurate images raise serious privacy concerns. These systems may also have limited range capability. Other systems, such as magnetic gradiometers, are more sensitive than current metal detectors and competitively priced. However, they still can only detect weapons made of ferromagnetic materials and have range issues [1]. X-ray backscatter has proven of interest but any increase in human dose may be unacceptable and the range is very limited. Hand manipulation is effective but puts the examiner at risk and is a slow process that is an invasion of privacy. Direct ultrasonics has been applied and found to be lacking in ability to discriminate weapons, especially smaller ones. Although the medical use of ultrasonic detection is well developed, it relies heavily on the strong coupling of an ultrasonic source to the body of interest, either through direct coupling, or a liquid or gel. The use of acoustical heterodyning in air is relatively new and its applications have been focused on entertainment systems.
[1] N. G. Paulter. “Guide to the Technologies of Concealed Weapon and Contraband Imaging and Detection.” NIJ Guide 602-00. National Institute of Justice Law Enforcement and Corrections Standards and Testing Program. February 2001.
[2] F. Felber, N. Wild, S. Nunan, D. Breuner, and F. Doft. “Handheld Remote Concealed Weapons Detector.” National Institute of Justice Final Technical Report J200-99-0032/3031. February 1999.
[3] F. S. Felber, C. Mallon, N. C. Wild, and C. M. Parry. “Ultrasound sensor for remote imaging of concealed weapons.” In SPIE Proceedings Vol. 2938, Command, Control, Communications, and Intelligence Systems for Law Enforcement, Eds: D. Spector, and E. M. Carapezza, pp. 110-119, November, 1996.
[4] F. Felber, N. Wild, S. Nunan, D. Breuner, and F. Doft. “Handheld ultrasound concealed weapons detector,” In SPIE Proceedings, Vol. 3575, Conference on Enforcement and Security Technologies, pp. 89-98, November, 1998.
[5] Jaycor web pages, http://wwwjaycor.com/jaycor_main/web-content/eme_sens_acoustic.html, http://wwwjaycor.com/jaycor_main/web-content/eme_sens_acoustic_apps.html, http://wwwjaycor.com/jaycor_main/web-content/eme—sens_acoustic_tech.html, http://wwwjaycor.com/jaycor_main/web-content/eme_sens_ultra.html.
[6] Nacci, P. L. and Mockensturm, L. “Detecting Concealed Weapons: Technology Research at the National Institute of Justice.” Corrections Today, Vol. 63, No. 4, July, 2001.
[7] M. Fatemi and J. F. Greenleaf. Vibro-acoustography: An imaging modality based on ultrasound-stimulated acoustic emission. Proc. Natl. Acad. Sci. 96: 6603-6608 (1999).
[8] M. Fatemi and J. F. Greenleaf. “Ultrasound-Stimulated Vibro-Acoustic Spectrography,” Science, Vol. 280, No. 3, April, 1998.
[9] Greenleaf, et al, U.S. Pat. No. 5,903,516, “Acoustic force generator for detection, imaging and information transmission using the beat signal of multiple intersecting sonic beams.”
[10]Greenleaf, et al, U.S. Pat. No. 5,921,928, “Acoustic force generation by amplitude modulating of a sonic beam.”
[11] Fatemi, et al, U.S. Pat. No. 5,991,239, “Confocal Acoustic Force Generator.”
[12] Fatemi, et al, U.S. Pat. No. 6,511,429, “Ultrasonic methods and systems for reducing fetal stimulation.”
[13] Fatemi, et al, U.S. Pat. No. 6,709,407, (also see U.S. patent application Ser. No. 20030083595), “Method and apparatus for fetal audio stimulation.”
[14] Averkiou, U.S. Pat. No. 6,440,075, “Ultrasonic diagnostic imaging of nonlinearly intermodulated and harmonic frequency components.”
[15] Averkiou, U.S. Pat. No. 6,544,182, “Ultrasonic nonlinear imaging at fundamental frequencies.”
[16] Doukas, U.S. Pat. No. 6,428,532, “Selective tissue targeting by difference frequency of two wavelengths.”
[17] Norris, U.S. Pat. No. 5,889,870, “Acoustic heterodyne device and method.”
[18] Norris, et al, U.S. Pat. No. 6,577,738, “Parametric Virtual Speaker and Surround-Sound System.”
[19] Pompei, U.S. Pat. No. 6,775,388, “Ultrasonic transducers.”